A Stolperstein (pronounced [ˈʃtɔlpɐˌʃtaɪn] ^ⓘ; plural Stolpersteine; in English "stumbling block") is a ten-centimetre (3.9 in) concrete cube bearing a brass plate inscribed with the name and life dates of victims of Nazi extermination or persecution. The Stolpersteine project, initiated by the German artist Gunter Demnig in 1992, aims to commemorate persons at the last place that they chose freely to reside, work or study (with exceptions possible on a case-by-case basis) before they fell victim to Nazi terror, forced euthanasia, eugenics, deportation to a concentration or extermination camp, or escaped persecution by emigration or suicide. As of June 2023^[update], 100,000 Stolpersteine have been laid, making the Stolpersteine project the world's largest decentralized memorial.

The majority of Stolpersteine commemorate Jewish victims of the Holocaust. Others have been placed for Sinti and Romani people (then also called "gypsies"), Poles, homosexuals, the physically or mentally disabled, Jehovah's Witnesses, black people, members of the Communist Party, the Social Democratic Party, and the anti-Nazi Resistance, the Christian opposition (both Protestants and Catholics), and Freemasons, Spanish Republicans along with International Brigades soldiers in the Spanish Civil War, military deserters, conscientious objectors, escape helpers, capitulators, "habitual criminals", looters, and others charged with treason, military disobedience, or undermining the Nazi military, as well as Allied soldiers.

The name of the Stolpersteine project invokes multiple allusions. In Nazi Germany, an antisemitic saying, when accidentally stumbling over a protruding stone, was: "A Jew must be buried here". In a metaphorical sense, the German term Stolperstein can mean "potential problem". The term "to stumble across something", in German and English, can also mean "to find out (by chance)". Thus, the term provocatively invokes an antisemitic remark of the past, but at the same time intends to provoke thoughts about a serious issue. Stolpersteine are not placed prominently, but are rather discovered by chance, only recognizable when passing by at close distance. In contrast to central memorial places, which according to Demnig can be easily avoided or bypassed, Stolpersteine represent a much deeper intrusion of memory into everyday life.

Stolpersteine are placed right into the pavement. When Jewish cemeteries were destroyed throughout Nazi Germany, the gravestones were often repurposed as sidewalk paving stones. The desecration of the memory of the dead was implicitly intended, as people had to walk on the gravestones and tread on the inscriptions. The Stolpersteine provocatively hint at this act of desecration, as they lack any kind of defense against new acts of shame. While the art project thus intends to keep alive the memory, implying that improper acts could easily happen again, the intentional lack of defense against potential desecration also created criticism and concern. Some German cities like Munich still do not accept the setting of Stolpersteine, and look for alternative ways of commemoration instead.

Research about future Stolperstein locations is usually done by local school children and their teachers, victims' relatives, or local history organizations. The database of Yad Vashem in Jerusalem and the online database of the Mapping the Lives publication of the 1939 Germany Minority Census are used to search for names and residential addresses of Nazi victims.

When research on a particular person is completed, Demnig sets out to manufacture an individual Stolperstein. The person's name and dates of birth, deportation and death, if known, are engraved into the brass plate. The words Hier wohnte ... ('Here lived ...') are written on most of the plates, emphasizing that the victims of persecution did not live and work at any anonymous place, but "right here". The Stolperstein is then inserted at flush level into the roadway or sidewalk, at the individual's last known place of freely chosen residence or work, with the intention to "trip up the passer-by" and draw attention to the memorial.

The costs of Stolpersteine are covered by individual donations, local public fund raising, contemporary witnesses, school classes, or community funds. From the beginning of the project until 2012, one Stolperstein cost €95. In 2012, the price increased to €120. Each individual Stolperstein is still manufactured by hand, so that only about 440 of them can be produced per month. Today, it may take up to several months from the application for a new Stolperstein until it is finally installed.

Starting in 2005, Michael Friedrichs-Friedländer has partnered with Gunter Demnig to install about 63,000 Stolpersteine in 20 different languages. Friedrichs-Friedländer explained to a reporter that he has not changed the engraving process and all engraving continues to be completed by hand; this is purposeful, to prevent the process from becoming anonymous.